As we've established in your threads on wars, there can be multiple web modules. There can also be multiple EJB modules making the answer "as many as you want."

I'd like to take this opportunity to recommend looking at a book like "EJB 3 in Action" or the free Sun JEE tutorial to read about enterprise applications. I think it will answer a lot of questions and connect the dots. Which will be more useful that a lot of disconnected questions/answers.